fs: descend into sub directories when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/fs/ext4/Makefile b/fs/ext4/Makefile
index 0f5d399..8d15bda 100644
--- a/fs/ext4/Makefile
+++ b/fs/ext4/Makefile
@@ -9,5 +9,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_FS_EXT4) := ext4fs.o ext4_common.o dev.o
+obj-y := ext4fs.o ext4_common.o dev.o
 obj-$(CONFIG_EXT4_WRITE) += ext4_write.o ext4_journal.o crc16.o